1. Field of the Invention
This invention relates to a sound reproducing apparatus such as a stereo reproducing apparatus and a voice reproducing apparatus, and more particularly to a sound reproducing apparatus in which an initial reflected sound and a reechoed sound can be added.
2. Description of the Related Art
FIG. 2 of the accompanying drawings illustrates a typical conventional apparatus for adding an initial reflected sound and a reechoed or reverberated sound. In FIG. 2, control signals are supplied to an initial reflective sound generator 3 and a reechoed sound generator 4 from a control signal generating unit 2 under the control of an operating unit 1. In an adder 5, the outputs of these two generators 3, 4 are added to a stereo signal inputted from an input terminal 6.
The operating unit 1 is composed of a plurality of adjusting knobs for adjusting a delay time of the initial reflected sound, an addition ratio, a delay time of reecho or reverberation, a reduction ratio, etc.; as the individual adjusting knob is turned, the resistance value of a variable resistor varies The initial reflective sound generator 3 is composed of a delay unit 3a for giving a delay time to a first initial reflected sound, a reducer 3b for giving a mixing ratio, another delay unit 3c for giving a delay time to a second initial reflected sound, and another reducer 3d for giving a mixing ratio. This illustrated example is the case in which a sound signal is a one-channel signal. Alternatively, in the sound signal could be a two-channel signal like a stereo signal. Designated by 7 is an output terminal from which a stereo signal or a voice signal is to be outputted.
In operation, a sound signal, i.e., one channel of a stereo signal inputted from the input terminal 6, is branched into two partial signals. One partial signal is delayed a predetermined time and is reduced at a predetermined rate in the initial reflective sound generator 3. The other partial signal is shaped into a reechoed sound having a predetermined delay time and a decay gradient by the reverberation generator 4. The individual reechoed signals processed in the initial reflective sound generator 3 and the reverberation generator 4 are added to the original sound signal at the adder 5 to form a composite signal, which is outputted from the output terminal 7.
At that time, for instance, when the user turns the delay-time-of-initial-reflected-sound adjusting knob at the operating unit 1, the resistance value of its associated variable resistor varies to create a voltage proportional to the resistance value. This voltage is inputted to the control signal generating unit 2. In the control signal generating unit 2, a control signal is given to the delay unit 3a which then generates a delay time control signal, depending on the voltage, to vary the delay time of the initial reflected sound, the mixing ratio of the initial reflected sound, and the decay time and the decay gradient of the reechoed sound. For setting an optional value, the user turns a correspond adjusting knob to vary the mixing ratio, the delay time, decay reduction gradient or other parameter.
Thus in the conventional apparatus, the user must to set the initial reflected sound and the reechoed sound by manipulating the various adjusting knobs by himself/herself. The setting and adjusting of the initial reflected sound and the reechoed sound are made dependent upon the user's hearing while a sound source such as a music or a voice is being reproduced. During that time, the user must repeat attempted adjustments by trial and error, actually listening the sound source. Further, since the most preferred values of the initial reflected sound and the reechoed sound vary remarkably depending on the kind of a music, it is necessary to make an adjustment for each and every item of played music. When the adjustment is not proper, an instrument sound would be apparently doubled and is hence acoustically unnatural, thus giving an uncomfortable auditory impression to the listener.
According to some studies on acoustics, it is a common knowledge that a reflected sound and subsequent reflected sounds arriving behind a direct sound are significantly influential on the sense of hearing. In another acoustics study, a composite sound field, which is composed of a direct sound and a single reflected sound as a music and a voice are reproduced by a speaker, is evaluated in terms of preference (comfortability on the hearing sense of a human being). According to the principles of acoustics, when the level of a reflected sound was varied in the range of .+-.6 dB of a direct sound, upon having obtained the auto-correlation function l.rho.(.tau.) of the regulated sound source signal, it was found that the most preferred delay time .vertline.l.sub..rho. (.tau.).vertline. of its reflected sound corresponded to a time equivalent to 1/10 of the level A.sub.1 of the first reflected sound. FIG. 3 is a graph showing the relationship between the time .tau..sub.d (the x-axis), when this preference .vertline.l.sub..rho. (.tau.).vertline. is equivalent to 1/10 of the level A.sub.1 of the first reflected sound, and the delay time .tau..sub.m (the y-axis) of the single reflected sound, when the preference is maximal. The illustrated range shows the delay time when the preference is lower than the maximal value by 0.1. In FIG. 3, a circle indicates A.sub.1 =6 dB; a dot, A.sub.1 =0 dB; a square, A.sub.1 =-6 dB. Specifically, assuming that the time in which .vertline.l.sub..rho. (.tau.).vertline. is 1/10 of l.sub..rho. (0) is called .tau..sub.e (0.1), .tau..sub.d =.tau..sub.e (0.1) where A.sub.1 =0 dB. It is apparent from FIG. 3 that .tau..sub.d is very coincident with the delay time .tau..sub.m of the single reflected sound when the preference is maximal.
Further, it was reported that the auto-correlation function of a sound source also in a close connection with the most preferred reverberation time. The results of measurement are shown in FIG. 4, in which the x-axis is the above-mentioned .tau..sub.e (0.1) the y-axis is the central value [T.sub.sub ].sub.d of the preferred reverberation time. Here "reverberation time" is defined as the time necessary until the signal of the reverberation part decreases to 60 dB, rather than the time in which the direct sound decreases by 60 dB. In FIG. 4, each of A, B and E indicates the case of a music, while S indicates the case of a voice; it is possible to obtain an approximation by the function EQU [T.sub.sub ].sub.d .apprxeq.(23.+-.10).tau..sub.e (0.1).
In the same method of measurement, the most preferred conditions, for the delay time of the second reflected sound in the sound field where two reflected sounds exist, was obtained as follows: The delay time of the first reflected sound is .DELTA.t.sub.1 =20, 30, 40 ms; the differential between the delay time of the second reflected sound .DELTA.t.sub.2 and the delay time of the first reflected sound .DELTA.t.sub.1 is .DELTA.t.sub.2 -.DELTA.t.sub.1 =10, 20, 30 ms; and these are combined. As a result of the same measurement of preference, preference curves shown in FIGS. 5(a) and 5(b) were obtained. FIG. 5(a) shows the preference curves as measured where the levels of the first and second reflected sounds were -4.2 dB and -6.2 dB, compared to the direct sound. FIG. 5(b) shows the preference curves as measured where the levels of the first and second reflected sounds were equal to the level of the direct sound. From FIGS. 5(a) and 5(b), it is apparent that if the delay time difference of the first and second reflected sounds .DELTA.t.sub.2 -.DELTA.t.sub.1 is about 0.8 times of the maximal delay time of the first reflected sound [.DELTA.t.sub.1 ].sub.p, the preference is largest. Consequently it is understood that the most preferred delay time of the second reflected sound [.DELTA.t.sub.2 ].sub.p may be approximately expressed by the following equation: EQU [.DELTA.t.sub.2 ].sub.p .apprxeq.1.8[.DELTA.t.sub.1 ].sub.p.
In the foregoing conventional apparatus, the most preferred delay time of the initial reflected sound, the most preferred reverberation time, etc. can be obtained from the auto-correlation of the sound source signal. Generally, however, a great amount of calculation is necessary to calculate the auto-correlation, and therefore it is difficult to finish the calculation within a practical time in a public welfare equipment such as a stereo reproducing apparatus.